1. Field of the Invention
The present invention relates generally to a method of fabricating a compound film, and more particularly, to a method of fabricating a compound film composed of a plurality of elements in which the vapor pressure of the particular element is higher than the vapor pressures of the other elements.
2. Description of the Background Art
It has been expected that a Tl system oxide superconductor such as a TlBaCaCuO superconductor (a thallium-barium-calcium-copper-oxygen system superconductor; a TBCCO system superconductor) is applied in a microwave device and a milliwave device because it is low in resistance on the surface (surface resistance) of its crystal through which a current flows, has a small loss in a high-frequency band such as a microwave band or a millimeter wave band, and has a critical temperature exceeding 100K so that it can be used at high temperatures.
In fabricating a superconductive film of high quality, an in-situ process for crystallizing a substrate during film formation while heating the substrate in a reaction chamber (film formation in the same reaction chamber) is presupposed. In the TlBaCaCuO superconductor, however, the vapor pressure of Tl is high. Therefore, Tl evaporates and escapes before TlBaCaCuO reaches a crystallization temperature by heating the substrate during film formation. Consequently, the composition ratio of Tl in a film which is being formed is decreased at the crystallization temperature. Therefore, a superconductive film having sufficient superconductive characteristics is not obtained.
When a TlBaCaCuO superconductive film is fabricated, therefore, a precursor film (a precursor) having an amorphous structure including Tl, Ba, Ca, Cu, and O is formed at room temperature, and the precursor film is heat-treated at a crystallization temperature of approximately 800 to 900.degree. C. Also in the case of the heat treatment, Tl evaporates, so that the composition ratio of Tl is decreased. In order to compensate for Tl which evaporates, therefore, a pellet for compensation (a material supply member) composed of Tl, Ba, Ca, Cu and O, together with the precursor film, is arranged in a sealed container, for example, to heat-treat the precursor film. Consequently, the sealed container is filled with vapors of Tl, so that Tl in the precursor film is compensated for.
In the conventional method of fabricating a TlBaCaCuO superconductive film, however, the precursor film must be heat-treated at a high temperature of approximately 800 to 900.degree. C. in order to crystallize the precursor film, so that the surface of the TlBaCaCuO superconductive film is roughened.